The Influence of Oxygen Content on the Thermal Activation of Hematite Nanowires

TLDR: This is the first demonstration of highly photoactive hematite nanowire arrays at a relatively low activation temperature without a dopant element and shows substantially enhanced photoactivity compared to the pristine hematites prepared in air.

Abstract: A promising photoelectrode material for solar-driven water splitting, hematite (a-Fe2O3) is non-toxic, abundant, chemically stable, low-cost, and has a bandgap of approximately 2.1 eV, which accounts for a maximum theoretical solar-tohydrogen (STH) efficiency of 15%. This last property compares favorably with the most studied metal oxide materials for photoeletrochemical (PEC) water splitting, including TiO2, [6–10] ZnO, and WO3. [12–15] However, the reported STH efficiencies of hematite photoelectrodes are substantially lower than the theoretical value, owing to several limiting factors such as poor conductivity, short excited-state lifetime (< 10 ps), poor oxygen evolution reaction kinetics, low absorption coefficient, short diffusion length for holes (2–4 nm), and lower flat-band potential in energy for water splitting. Enormous efforts have been made to overcome these limitations of hematite, including the incorporation of oxygen evolving catalysts to reduce the kinetic barrier for water oxidation on the hematite surface, the development of nanostructures to increase the effective surface area and to reduce diffusion length for carriers, as well as the development of element-doped hematite for improving electrical conductivity and/or light absorption. Recently, we demonstrated that TiO2 nanowires thermally treated in hydrogen showed increased donor density and PEC performance as a result of the formation of oxygen vacancies. We anticipated that creating oxygen vacancy (VO), and thereby Fe, sites in hematite could significantly increase the conductivity of the material through a polaron hopping mechanism. Although VO can be created by sintering hematite in a reductive atmosphere such as hydrogen, it may introduce hydrogen as a dopant into the structure. Additionally, hematite can be easily reduced in hydrogen to produce magnetite (Fe3O4), which is photo-inactive. [27] Herein, we report an alternative method for the preparation of highly conductive and photoactive hematite through thermal decomposition of b-FeOOH in an oxygen-deficient atmosphere (N2+ air). The resulting hematite sample showed substantially enhanced photoactivity compared to the pristine hematite prepared in air. The oxygen content during thermal activation significantly affects the formation of VO and thereby the photoactivity of hematite nanowires for water oxidation. This is the first demonstration of highly photoactive hematite nanowire arrays at a relatively low activation temperature without a dopant element. Akaganeite nanowires were prepared through the hydrolysis of FeCl3 (0.15m) in an environment with a high ionic strength (1m NaNO3) and low pH value (pH 1.5, adjusted by HCl) at 95 8C for 4 h. The resulting yellow film on a fluorine-doped tin oxide (FTO) substrate was covered with nanowire arrays with an average diameter and length of 70 nm and 700 nm, respectively (Figure 1a). X-ray diffraction